@CDW,
You have not mentioned which mobile device, or the type of connection you require, but as examples for say an iPhone, you can get an adapter like this…

https://www.amazon.co.uk/dp/B07Q6K37BD (Lightning)

or this…

https://www.amazon.co.uk/dp/B082WG5VTK (USB-C)

…all depending of course on your mobile output port and connect it using a ‘3.5mm-to-3.5mm’ audio cable to the Move 2, using the Sonos audio dongle as shown in this link:

https://www.amazon.co.uk/dp/B07Q6K37BD

@CDW 

How exactly did you connect your phone to the speaker?

Move2 has a usb c connector and to use that for analogue line in you have to use the oroginal Sonos adapter. 3rd party analogue to usb c adapters won‘t work… and also a direct digital usb c connection isn’t possible. 

@Schlumpf I've got the original Sonos line-in usb c adapt for move 2. But that has a female 3.5mm Jack opening. So I'm assuming I need a 3.5m male connector that then attaches to my phone but I don't think sonos sells that?

@Ken_Griffiths I've connected my android device via a doubled ended USB c cable. But whilst that charges my phone sonos doesn't recognise as a line-in. 

The Sonos adapter is the only way to get a line in signal to Move2. USB C direkt connection as said doesn’t work. To connect devices that have an analogue line out you can use any kind of usual audio cable. But if your phone just has a digital connector like usb c or Apple lightning, you need a second adapter for the phone (probably sold by the phone manufacturer) that converts the digital signal to analogue line out. Imho a kind of conversion madness… DA-AD conversion…

For me using an iPhone 14 I need the Apple DA adapter (lightning to 3,5 female), a standard 3,5 male to male cable and the Sonos adapter. 
Not to be called user friendly.

The Move 2 requires the ‘ADC’ Sonos line-in adapter, then use the USB-C ‘DAC’ adapter (or similar, as mentioned in my first post). Just link the two with a 3.5mm to 3.5mm audio cable - that should work for you.
My Son did once use a similar wired setup with his iPhone 11 linked to Era 300 speakers, but now mostly uses Bluetooth, as it’s very reliable in his case and has no issues… but if you want to use line-in, then the setup I’ve described will work.

You could also take a look at why the Bluetooth connection is unstable. Is something wrong with the speaker, or your phone?

@CDW 

I just can agree to the other respondents… seems you’re having some general skepticism about bluetooth. For me in nearly every case the Sonos BT connection works quite well. If I were you, I would give it a chance. 

Only Bluetooth issues I’ve had are too close, under 3 feet, or too far, past 20 feet, giving an iffy connection.

Thanks all. The  3x different cable attachments was the solution. 

CDW, how did you switch the speaker to Line-in without wifi. I use mine out in parks with a DJ setup and have all the proper cables. 50% of the time though I can’t get it to switch to Line-in mode since I can’t use the app. It eventually always works but I can’t figure out ultimately what is making it switch to Line-in. Do you have any idea what buttons actually trigger it to work?

@SF_steve 

It should work, if autoplay is set active to moves room itself. 
Otherwise for manually switching to line in source you have to use the Sonos app. This could be done by using a mobile wifi router or hotspot the speaker and the device running Sonos app are connected to. 

Re connecting two Move 2s as a stereo pair, but NOT via wifi, has anybody investigated running left/right mono signals (split from a stereo signal source) into the two individual line-in connections?  Would this keep proper sync between the left/right channels, or is there some Sonos ‘funny business’ that would not guarantee the sync?

This is admittedly a very specific use case workaround to being unable to stereo pair two Move 2s via bluetooth.   But it would permit using WIFI (as designed) for stereo music playback, AND also as a direct stereo line out setup from a outdoor projector (or via bluetooth to a bluetooth stereo receiver). 

Sure…. I would lose the wireless aspect with the 2x line in setup, but it would mean that Moves could do double duty, and therefore be a more attractive proposition.  As the Move 2s currently stand they only really function as an outdoor music solution, not a general outdoor AV solution.

Any insight greatly appreciated.

So you’re trying to get around the fact that your Bluetooth generating device only puts out a single Bluetooth signal, and not two separate signals?

I can’t think of any particular reason why this wouldn’t work, although it would be extremely odd to try. Both Sonos line ins would be subject to the same 75ms delay, so they should be in sync, assuming the input is. But this is theoretical, I’ve certainly not tried this, nor would I want to. I’d much prefer to get a travel router and set up the two Sonos Moves to connect to it. 

@DukeLA 

I agree with @Airgetlam … in theoretical it could work though I I‘m not sure if both will stay in sync because it’s not a real sync control running, they just probably start playback at the same time. 
Also that would need an uncomfortable cable-adapter combination from L/R RCA stereo to 2x mono 3,5mm male plus 2x Sonos line in adapter. Depending on the distance between the two speakers that imo would be some cable salad.